7. ISOTOPIC COMPOSITION OF CH₄, CO₂ SPECIES, AND SEDIMENTARY ORGANIC MATTER WITHIN SAMPLES FROM THE BLAKE RIDGE: GAS SOURCE IMPLICATIONS¹

C.K. Paull,^{2, 3} T.D. Lorenson,⁴ W.S. Borowski,^{2, 5} W. Ussler III,² K. Olsen, ^{2, 6} and N.M. Rodriguez^{2, 5}

ABSTRACT

The isotopic characteristics of CH₄ (¹³C values range from -101.3 to -61.1 PDB, and D values range from -256 to -136 SMOW) collected during Ocean Drilling Program (ODP) Leg 164 indicate that the CH₄ was produced by microbial CO₂ reduction and that there is not a significant contribution of thermogenic CH₄ to the sampled sediment gas from the Blake Ridge. The isotopic values of CO₂ (¹³C range -20.6 to +1.24 PDB) and dissolved inorganic carbon (DIC; ¹³C range -37.7 to +10.8 PDB) have parallel profiles with depth, but with an offset of 12.5. Distinct downhole variations in the carbon isotopic composition of CH₄ and CO₂ cannot be explained by closed-system fractionation where the CO₂ is solely derived from the locally available sedimentary organic matter (¹³C -2.0 ± 1.4 PDB) and the CH₄ is derived from CO₂ reduction. The observed isotopic profiles reflect the combined effects of upwards gas migration and decreased microbial activity with depth.

¹Paull, C.K., Matsumoto, R., Wallace, P.J., and Dillon, W.P. (Eds.), 2000. Proc. ODP, Sci. Results, 164: College Station, TX (Ocean Drilling Program).
²Geology Department, University of North Carolina, Chapel Hill, NC 27599-3315, U.S.A.
³Present address: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039-0628, U.S.A. paull@mbari.org
⁴Pacific Marine Geology, U.S. Geological Survey, Menlo Park, CA 94025, U.S.A.
⁵Present address: Exxon Exploration Company, P.O. Box 4778, Houston, TX 77210-4778, U.S.A.
⁶Present address: Marine Sciences, State University of New York, Stony Brook, NY 11794, U.S.A.

Date of initial receipt: 30 April 1998
Date of acceptance: 12 January 1999
Ms 164SR-207